Accuracy of UTM Coordinate System
I have been told that the UTM coordinate system is only accurate to 1 meter. Is this true? My understanding is that coordinate system accuracy is only expressed as a ratio such as 1 part in 10000.
Mapping Center Answer:
I think there might be two concepts being confused here. One is the accuracy of data collection or digitization. That would be expressed in terms of "accurate to 1 meter". This reflects the accuracy of the digital feature relative to its known location on the ground. For example, some recreation grade GPS units will specify that the accuracy of positioning is to within 10 meters, but some professional grade GPS units can actually collect points to sub-meter accuracy. Here is an ArcUser article that discusses this concept in more depth:
The other idea is the precision of the mathematics behind the map projections. For a map that uses the UTM coordinate system, the underlying map projection is a Transverse Mercator projection. U UTM zone has a scale factor of 0.9996 along its central meridian, or four parts in 10,000, which is equivalent to 1:2,500. That is to say, given the math behind this projection, if you measure 2,500 "somethings" (e.g., feet, meters, inches) within the UTM zone, it will not be more than 1 "something" (e.g., foot, meter, inch) from its true length. This is, as you note, a unitless ratio.
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